1. Field of the Invention
The present invention relates to a manufacturing method for an auger adapted for use in an auger type ice making machine, various kinds of extruding machines or the like.
2. Description of the Prior Art
An auger of this kind has an auger body integrally formed thereon with a spiral blade and formed at its opposite ends with a pair of shaft portions. In the auger, it is required to enhance anti-corrosiveness and strength of the auger body, and it is also required to enhance anti-abrasiveness and hardness of the shaft portions.
To satisfy the requirements, ferritic stainless steel, martensitic stainless steel, austenitic stainless steel or the like has been used as a base metal of the auger, and an anti-abrasive and anti-corrosive alloy containing, as main components, Co, Cr, Mo and Ni mixed at a predetermined ratio has been built up by plasma-arc welding in each annular recess formed on the shaft portions and finished by grinding to form a journal portion on the respective shaft portions.
To further enhance the strength and hardness of the auger, it is desirable that precipitation hardening stainless steel is used as a base metal of the auger. In use of the precipitation hardening stainless steel, it is, however, indispensable that the base metal is subjected to aging heat treatment to enhance the strength and hardness of the auger. This results in an increase of the manufacturing cost of the auger.
It is, therefore, a primary object of the present invention to provide a manufacturing method for an auger in which precipitation hardening stainless steel is used as a base metal of the auger without being subjected to traditional aging heat treatment but instead, being subjected to an aging heat treatment caused, in part, by pre-heating before applying welding heat during a plasma-arc welding process to enhance the strength and hardness of the auger at a low cost.
According to the present invention, this object is attained by providing a manufacturing method for an auger composed of an auger body integrally formed thereon with a spiral blade and at its opposite ends with a pair of shaft portions, comprising the steps of forming annular recesses on the shaft portions in a cutting process respectively, building up an anti-abrasive and anti-corrosive alloy by plasma-arc welding in the annular recesses of the shaft portions and finishing each built-up portion of the alloy in a grinding process to form a journal portion on the respective shaft portions of the auger, wherein precipitation hardening stainless steel used as a base metal of the auger body (without having been yet subjected to aging heat treatment for precipitation hardening) and is preheated for a predetermined time prior to the plasma-arc welding of the alloy and thereby subjected to an effective aging heat treatment caused by the combined effects of preheating and welding heat during the plasma-arc welding process.
In a practical embodiment of the present invention, it is preferable that the precipitation hardening stainless steel used as the base metal essentially consists of 17Cr, 4Ni and 4Cu or 17Cr, 7Ni and 1Al. In the plasma-arc welding process, it is desirable that the precipitation hardening stainless steel is preheated at a temperature of 300xc2x0 C.-350xc2x0 C. for twenty minutes-eighty minutes.